This application pertains to the art of control systems, and more particularly to microprocessor based systems for controlling an energy source in response to sensed changes in parameters affected by the energy source.
The invention is particularly applicable to a clothes dryer dryness detection system and will be described with specific reference thereto. However, it will be appreciated that the invention has broader applications such as other control systems where changes in relative humidity or temperature are monitored as a control parameter and may be advantageously employed in such other environments and applications.
Clothes dryers are relatively well-known and common household appliances. They are automatic machines in the sense that the operator need only load the dryer and the dryer will turn itseft off after expiration of a preset drying time. An option on such dryers which has become popular recently is an automatic dryness control circuit which can sense whether the clothes in the dryer are actually dry instead of merely running for a preprogrammed time length. This further relieves the operator of the worry of either over-drying or under-drying of a clothes load, which can of course always vary depending upon the size and content of the load. The improved detection of dryness of the load is the overall objective of the subject application.
Dryness detection circuits for clothes dryers have heretofore comprised primarily two types. The first type employs contact traces in the rotating drum of the dryer, while the second employs a bi-metal thermostat to detect exit air temperature.
The trace contact systems comprise a hybrid (electromechanical) control system where electrical tracings in the dryer drum are intended to actually contact clothes contained therein. Opposed traces are set relatively close together so that if a wet item in the load makes contact with it a closed circuit connection is made across the traces. The control theory of this system is that every time a continuity occurs across the traces, a circuit will count the number of circuit closing as pulses which then can be communicated to a signal conditioning circuit. Based upon the number of pulses that are counted within a predetermined period of time, the relative dryness of the load in the dryer is determined. Typically, the frequency of the pulses are used to condition a signal to charge a capacitor, which, as long as it is maintained at a threshold, indicates that the clothes in the load are not yet dry. When the pulse frequency fails to maintain the capacitor threshold, a timer is initiated to count down a predetermined timeout period for the end of the drying cycle.
Another control system also employing traces merely counts the number of pulses with a microprocessor circuit and uses a predetermined algorithm to compare the counted pulses within a set period to a stored "dryness table" in a memory. In other words, a user merely presets the relative level of dryness desired for the load and, based upon the counted pulses of circuit closures across the traces, in comparison with the predetermined table, the relative dryness is detected.
In accordance with the second type of control system, contact traces are avoided in the drum and a thermostat senses the temperature of the exit air. Experience with clothes dryers has show that for a typical drying operation, a plot of time versus temperature shows that there will be a long plateau where the temperature is fairly constant during the drying operation of the clothes. After the moisture has been substantially evaporated so that the clothes are almost dry, the temperature on the plot will substantially increase. Exit air temperature systems will detect this inflection in the exiting air temperature and will also then turn on a timeout timer for the timeout of the drying cycle.
The problems which have been found with regard to the use of these two prior systems basically fall into ones of cost and reliability. The home appliance industry is so cost competitive that even seemingly minor reductions in component costs can present a substantial advantage in high volume product marketing. Suppliers of the components to the assemblers and manufactures of such dryers are under constant pressure to continually maintain or reduce the costs of such supplied components. Secondly, reliability in operation, both from longevity and durability standpoint, as well as an accuracy standpoint is of very high concern. The required dryness time to be employed by a clothes dryer will vary with load size, type of fabric, amount of moisture in the load, rate of evaporation, the way the clothes tumble and the amount of air flowing in the load, as well as the ambient room temperature. Any successful dryness detection system must be able to adapt to these varying conditions. Predetermined timeout tables are usually merely an experimentally based averaging of these varying conditions and cannot possibly encompass all possible condition sets. Reliability thus suffers. In addition, pulse counting of trace wire continuities is inherently unreliable since wet clothes in the dryer may simply miss the contacts enough to give an improper indication of clothes dryness.
Thermostat based systems for detecting temperature also have a problem with reliability in that the simply do not measure the dryness of the clothes. Rather, they measure a parameter which is hopefully representative of clothes dryness but due to the vagaries of the varying conditions noted above, such a system may not be as accurate as desired, particularly, one based on an averaging of predetermined experimental results.
Lastly, all prior art systems have the problem of cost. For those systems which employ printed circuit boards, as the systems above do, such items are relative high-cost devices, not only for their inherent cost themselves, but also for the cost of installation and assembly.
The present invention contemplates a new and improved control system and method which overcomes the above referred to problems and others to provide a new clothes dryer dryness detection system which is simple in design, economical to manufacture, readily adaptable to a plurality of load conditions including load size, type of fabric, amount of moisture in the load, rate of evaporation, varying clothes tumble, the amount of air flowing through the load and ambient temperature, and thus provides improved efficiencies in operation and reductions in manufacturing costs.